1. Field of the Invention
The present invention relates to a polarization mode dispersion (PMD) generating apparatus for generating PMD, a PMD-compensating apparatus for compensating PMD and a PMD-emulating apparatus for generating pseudo PMD, and methods therefor.
2. Description of the Background Art
One of the factors restricting the communication performance of high-speed optical communications is distortion of the temporal waveform of optical pulses caused through propagation of the optical pulses carrying an optical signal over an optical fiber transmission line. One of the factors in the distortion is polarization mode dispersion (PMD). PMD is caused by the birefringence structure of an optical fiber transmission line. When optical pulses propagate over an optical fiber, the birefringent property causes a propagation time difference, i.e. differential group delay (DGD), between the orthogonal polarization components of an optical pulse carrier wave. The phenomenon is PMD.
The magnitude or degree of PMD caused on an optical fiber transmission line may be represented by a PMD coefficient [ps/km1/2]. According to the Recommendation of the ITU-T (International Telecommunication Union Telecommunication Standardization Sector), the PMD coefficient of a standard single-mode fiber is recommended to be equal to or less than 0.2 ps/km1/2.
The average value of DGDs caused in an optical fiber actually installed may be calculated by multiplying its PMD coefficient by a transmission distance. More correctly, (Average value of DGD [ps])=(PMD coefficient [ps/km1/2]) (Square root of transmission distance [km1/2]). For example, for a fiber transmission line of a single-mode optical fiber having a PMD coefficient of 0.2 ps/km1/2, the average DGD caused across the distance of 100 km is equal to 2 ps on the basis of (0.2 ps/km1/2)×(1001/2 km1/2)=(0.2×10) ps.
Generally, older optical fiber transmission lines installed in earlier ages tend to raise the PMD coefficients thereof. It was reported that some of optical fiber transmission lines installed in the 1980's had PMD coefficients reaching even up to 5 ps/km1/2. Recently, optical fibers having PMD coefficients equal to or less than 0.02 ps/km1/2 have been developed.
When expanding optical fiber transmission line networks, in consideration of, for example, economic restriction imposed on the construction cost, such tactics may often be taken that optical fibers are laid on the basis of the principle that aged optical fiber transmission lines installed are to be utilized with new optical fiber transmission lines added thereto. Therefore, optical communications using optical fiber transmission line networks expanded in this way require a technique for reducing the influence of the PMD with a premise taken into account on an optical signal being transmitted over optical fiber transmission lines having large PMD coefficients.
A DGD value estimated from a PMD coefficient is a temporal average value, and has the nature of varying with time. Furthermore, the PMD of an optical fiber forming an optical fiber transmission line, namely PMD vector described below, has its magnitude and direction not constant in the direction of the transmission axis of the optical fiber but randomly varying depending on the distance. Thus, under the circumstances, such an optical fiber can be considered as configured by many short lengths of polarization-maintaining (PM) optical fiber connected in series with the PMD vector of each short length of optical fiber varying randomly. In other words, the optical fiber can be considered as configured by a plurality of divided sections corresponding to the short lengths of PM fiber interconnected in the optical waveguide direction so that each short length of PM fiber has its PMD vector different at random.
Now, the PMD vector is defined as a vector having its magnitude representing the magnitude of a DGD and its direction parallel to a unit Stokes vector in a principal state of polarization.
If the reciprocal of the DGD value of an optical fiber transmission line is larger than the spectrum bandwidth of an optical pulse signal, the influence of higher-order PMD cannot be negligible. Higher-order PMD is known as a phenomenon in which the principal state of polarization (PSP) of an optical fiber changes with respect to the frequency of optical signal or wavelength. The phenomenon is called depolarization. A change in the PSP vector is represented as a rotation of the terminal point of a PMD vector on the Poincare sphere. A difference of the propagation speed between an optical electric-field component parallel to a fast axis and an optical electric-field component parallel to a slow axis is called a polarization-dependent chromatic dispersion (PCD).
The magnitude and direction of a PMD vector generally depend on the wavelength of an optical carrier wave. However, if the spectrum bandwidth of an optical signal is negligible, the PMD vector needs to be treated only for the first-order PMD that is a PMD component free of wavelength dependency. However, if the spectrum bandwidth of an optical signal is not be negligible, the PMD vector needs to be treated also for higher-order PMD that is a PMD component depending on the wavelength.
The case where the spectrum bandwidth of an optical signal is negligible for higher-order PMD is directed to a case where the PMD coefficient in itself of an optical fiber transmission line is small, or where the reciprocal of a DGD value that is possibly be caused on the transmission line is sufficiently smaller than its optical signal band.
Higher-order PMD can also be described as below. When an optical pulse propagates over an optical fiber transmission line, among spectrum components of the optical pulse, a shorter- and a longer-wavelength component are different from each other in direction of the fast and slow axes. In detail, when the waveguide direction of an optical fiber transmission line is represented by the z-axis, the directions of the fast and slow axes of the optical fiber have dependency on the z-axis, so that the directions of the fast and slow axes of the entire transmission line are dependent upon the respective wavelength components. Additionally, the DGD value is also dependent upon the respective wavelength components so that the temporal waveform of the optical pulse deforms complicatedly. As described above, a PMD caused by variations in the directions of the fast and slow axes and by a variation in a DGD value depending on the wavelength will be a higher-order PMD.
The first-order PMD is based on a concept of taking no account of wavelength dependency of PMD. The second-order PMD is a phenomenon in which the wavelength dependency varies at a constant rate. Higher-order PMD is a phenomenon in which the wavelength dependency varies at a more complicated rate instead of a constant rate.
In order to increase a transmission speed, the time width, or duration, of an optical pulse needs to be decreased. If the duration of an optical pulse is decreased, the bandwidth of the spectrum of the optical pulse is increased. Therefore, to estimate the influence of PMD on an optical fiber transmission line in an optical communications system having a high transmission speed, it is important to consider higher-order PMD in addition to the first-order PMD.
As described above, PMD to be considered has its upper limit dependent on the state, given as a PMD coefficient, of an optical fiber transmission line and the length of the bit period of an optical signal. Thus, a solution for alleviating the influence of PMD on an optical fiber transmission line is required. To configure and operate an optical communications system, a solution for testing the tolerance to PMD caused on an optical fiber transmission line is required. However, PMD on an optical fiber transmission line is randomly raised by uncertain factors such as external environment, and hence difficult in reproduction. Thus, an inspection apparatus for generating pseudo PMD on an optical fiber transmission line is required. In the context, such an inspection apparatus may be referred to as a PMD-emulating apparatus or emulator.
Among PMD-compensating methods for alleviating the influence of PMD on an optical fiber transmission line, multilevel modulations have been widely researched which can decrease the symbol rate of a signal without changing a bit rate. The multilevel modulations are exemplified by DQPSK (Differential Quadrature Phase Shift Keying), QAM (Quadrature Amplitude Modulation), and OFDM (Orthogonal Frequency Division Multiplexing).
On a receiver side, there are known the optical compensating method and the electric compensating method. In the optical compensating method, PMD having its property inverse to that of PMD on an optical fiber transmission line or equalize PMD is generated by optical circuits configured by a combination of optical elements to compensate for PMD on the optical fiber transmission line. In the electric compensating method, a received optical signal is optoelectrically converted to equalize a waveform using a transversal filter, which is an analog electronics. Other known electric compensating methods are exemplified by a method for performing A/D (Analog-to-Digital) conversion with a sampling rate twice as high as the symbol rate and equalizing a waveform through digital signal processing by an FIR (Finite-Impulse-Response) filter.
The electric compensating method is excellent in high adaptive equalization speed against a fluctuation in PMD, but is restrictive in its operation depending on the symbol rate of a signal. The method based on digital signal processing requires a high-speed A/D converter and high-speed logics, and is therefore problematic in saving power consumption.
To compare system configurations between both methods, the optical compensating method has a problem that the size of hardware is larger than electronics required by the electric compensating method. The optical compensating method has other problems that a compensation operation speed is slower and that the apparatus is more expensive. However, in the optical compensating method, it is advantageous that the operation has low dependency on the bit rate and modulation format of a signal to be processed, and that the functional components consuming significant electric power are only a portion for physically driving optical elements, thus readily accomplishing low power consumption. Moreover, it is advantageous that a PMD-compensating apparatus for use in the optical compensating method can be used not only as an apparatus for compensating PMD on an optical fiber transmission line as described above but also as a PMD-emulating apparatus for emulating PMD generated on the optical fiber transmission line.
A PMD-generating apparatus for implementing the optical compensating method is configured by a combination of a polarization plane controller and a DGD generator as main constituent elements. Assuming that a DGD on an optical fiber transmission line is reduced to 10% or less of the bit rate period of a signal, the PMD-generating apparatus preferably has its DGD generator adapted to variably change the DGD to compensate for PMD in the range of high-speed signal. Due to the same reason, also when the PMD-generating apparatus is operated as a PMD-emulating apparatus, it is assumed that a high-speed signal is processed so as to preferably have its DGD generator adapted to variably change the DGD to compensate for PMD.
Known types of variable DGD generators capable of variably changing a DGD may include a type adapted to use a mobile mirror mechanically driven for providing difference in optical path length between orthogonal polarization modes and a type including a combination of a birefringent medium and a polarization plane rotation mechanism.
For example, Japanese patent laid-open publication No. 2003-228026 to Kazuhiro Ikeda proposes a variable group delay time imparting unit. Ikeda discloses a variable DGD generator adapted for imparting a predetermined group delay time twice to incident light and variably rotating the state of polarization in the period from the time of imparting the former group delay time to the time of imparting the latter group delay time. This kind of variable DGD generator includes an optical circulator, a Faraday rotator, a PM fiber or a birefringent medium, and a reflective mirror.
Lianshan Yan et al., “Programmable Group-Delay Module Using Binary Polarization Switching”, Journal of Lightwave Technology, Vol. 21, No. 7, pp. 1676-1684, July 2003, discloses a variable DGD generator in which plural birefringent media having DGD values related with each other by a power of two are connected through MO (Magnet Optic) switches operative to select the couple of eigen-axes of the birefringent media so as to change in binary the state of polarization between the eigen-axes to render the DGD vary.
Furthermore, Phua et al., “Variable Differential-Group-Delay Module Without Second-Order PMD”, Journal of Lightwave Technology, Vol. 20, No. 9, pp. 1788-1794, September 2002, discloses a method for using four birefringent media connected across variable phase shifters to variably generate a DGD while restraining a DGD generator itself from generating a higher-order PMD. An apparatus for implementing this method is one using a variable DGD generator including four birefringent media and three polarization rotation mechanisms.
Features to be required for a PMD-compensating apparatus and a PMD-emulating apparatus include, for example, higher operational speed, miniaturization, highly accurate PMD generation, long-standing stable operation, minimization of higher-order PMD, power saving, insensitivity to external environment, readiness in installation and control, and broader applicable range of wavelength. Particularly, a PMD-generating apparatus for common use in the PMD-compensating apparatus and the PMD-emulating apparatus requires features of capability of generating the first-order PMD with the second-order PMD restraint from being generated, higher operational speed, and readiness in installation and control.
Recently, a PMD tolerance test to a polarization multiplex signal has actively been researched. The PMD-emulating apparatus requires the capability of continuously changing the state of polarization (SOP) of light outputted when changing a PMD vector and similar to PMD generated on an installed optical fiber.
It was observed that PMD on an optical fiber transmission line varied at a speed of millisecond order. Therefore, a PMD-generating apparatus including a PMD-compensating apparatus and a PMD-emulating apparatus requires its operation speed applicable to the observed speed. However, the mechanical mobile mirror is limitative in adjusting speed of DGD to several ten milliseconds. Furthermore, the mobile mirror may be inferior in mobile portions possibly malfunctioning due to abrasion or vibration through a long-term operation. Thus, the PMD-generating apparatus using a DGD generator including a mechanical mobile mirror involves difficulty in long-term high speed and stable operation.
In a variable DGD generator disclosed in Lianshan Yan et al., in which the birefringent media are connected through the MO switches operative to select a couple of eigen-axes of the birefringent media so as to change in binary the state of polarization between the eigen-axes to vary a DGD, the generable DGD value is limited to discrete values depending on a combination of birefringent media. Therefore, in order to generate highly accurate PMD, the DGD value has to be changed in minute steps. Thus, many birefringent media and MO switches are needed. Additionally, a delay difference between the orthogonal eigen-axes is switched at a speed of picosecond order together with operation of varying a DGD, thereby the output SOP from the DGD generator abruptly changing. Furthermore, when switching the amount of DGD to be generated, the eigen-axes of plural birefringent media are not orthogonal to each other in a period from the start to the end of the switching. Therefore, higher-order PMD may be generated in that period, also being problematic.
In the solution disclosed in Ikeda stated earlier for continuously rotating the SOP between plural birefringent media to change a mode coupling state between the birefringent media, a DGD value observed in a specific wavelength can be continuously changed. However, the second-order PMD is generated, which is problematic.
A DGD value τ generated in the DGD generator and the magnitude |τω| represented by the absolute value of the DGD of the second-order PMD value, respectively, are given by expressions (1) and (2):τ=(τ12+τ12+2τ1τ2 cos 2θ)1/2  (1)|τω|=τ1τ2 sin 2θ  (2)where τ1 and τ2 are the magnitudes of DGDs of the respective two birefringent media, and 6 is a polarization plane rotation amount therebetween.
The solution disclosed in Phua et al., can continuously vary the magnitude of a DGD and reduce the generation of higher-order PMD. In the respective unit function blocks, a variable polarization rotator rotates the state of polarization to vary the DGD value according to the expression (1). With this operation, the second-order PMD given by the expression (2) is generated. However, the nature of the first-order PMD vector orthogonal to the second-order FMD vector in the Stokes space can be used to cancel the second-order PMD component while maintaining only the first-order PMD component.
That solution uses the symmetric structure of two unit function blocks having equal PMD vectors to cancel the second-order PMD component, and generates the first-order PMD component. Therefore, those unit function blocks are required to have the same optical property as each other. Thus, four birefringent media and two polarization plane rotators in total need to have the same optical property as each other.
Uniformity has to be assured of the DGD values in the respective birefringent media and the polarization plane rotation amounts of the polarization plane rotators. However, in practice, it is significantly difficult to select and lay birefringent media and polarization plane rotators satisfying the above conditions and to control the two polarization plane rotators so as to be equalized in optical property.
The variable DGD generator implementing the solution disclosed in Phua et al., has a couple of mode mixers adapted to be controlled so as to operate completely equivalently to each other. This requires, for example, accurate temperature adjustment and maintaining the relative relationship of phase between devices, resulting in many difficulties in control and installation. The second-order PMD in the wavelength of the optical carrier wave can be canceled. However, depending on a DGD generated in the birefringent medium, the free spectral range (FSR) is determined, thus limiting an applicable band. According to Phua et al., the FSR is given as 1/(2τ) where r is the absolute value of a PMD vector, i.e. the magnitude of a DGD generated in a birefringent medium.
Recently, there have been researched modulation formats such as OFDM, QPSK, and QAM which may occupy different frequency bandwidths although having the same transmission speeds. Therefore, a PMD-generating apparatus is expected which can variably change a compensation wavelength band and/or an emulation band.
The mechanical implementation can thus generate PMD having a significantly flat wavelength property, but is problematic in its insufficient response speed.
As described above, the conventional art can implement a PMD-generating apparatus that can continuously change a PMD value at a speed of millisecond order or less but is difficult in accomplishing a PMD-generating apparatus that can determine an applicable wavelength band.